Method of making a FeCrAl material and such material

ABSTRACT

A method of producing an FeCrAl material by gas atomization, and a high temperature material produced by the method. In addition to containing iron (Fe), chromium (Cr), and aluminium (Al) the material also contains minor fractions of one or more of the materials molybdenum (Mo), hafnium (Hf), zirconium (Zr), yttrium (Y), nitrogen (N), carbon (C) and oxygen (O). The smelt to be atomized contains 0.05-0.50 percent by weight tantalum (Ta) and less than 0.10 percent by weight titanium (Ti). Nitrogen gas (N 2 ) is used as an atomizing gas, to which an amount of oxygen gas (O 2 ) is added, the amount of oxygen gas being such as to cause the atomized powder to contain 0.02-0.10 percent by weight oxygen (O) and 0.01-0.06 percent by weight nitrogen (N).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of producing an FeCrAlmaterial, and also to such material.

2. Description of the Related Art

Conventional iron-based alloys containing typically Fe and 12-25% Cr and3-7% Al, so-called FeCrAl-alloys, have been found highly useful invarious high temperature applications due to their good oxidationresistance. Thus, such materials have been used in the production ofelectrical resistance elements and as carrier materials in motor vehiclecatalysts. As a result of its aluminum content, the alloy is able toform at high temperatures and in the majority of atmospheres animpervious and adhesive surface oxide consisting substantially of Al₂O₃.This oxide protects the metal against further oxidation and also againstmany other forms of corrosion, such as carburization, sulphuration, etc.

A pure FeCrAl alloy is characterized by a relatively low mechanicalstrength at elevated temperatures. Such alloys are relatively weak athigh temperatures and tend to become brittle at low temperaturessubsequent to having been subjected to elevated temperatures for arelatively long period of time, due to grain growth. One way ofimproving the high temperature strength of such alloys is to includenon-metallic inclusions in the alloy and therewith obtain aprecipitation hardening effect.

One known way of adding said inclusions is by a so-called mechanicalalloying process in which the components are mixed in the solid phase.In this regard, a mixture of fine oxide powder, conventionally Y₂O₃, andmetal powder having an FeCrAl composition is ground in high energy millsover a long period of time until an homogenous structure is obtained.

Grinding results in a powder that can later be consolidated, forinstance by hot extrusion or hot isostatic pressing to form a completelytight product.

Although Y₂O₃ can be considered to be a highly stable oxide from athermodynamic aspect, small particles of yttrium can be transformed ordissolved in a metal matrix under different circumstances.

It is known that in a mechanical alloy process yttrium particles reactwith aluminum and oxygen, therewith forming different kinds ofY—Al-oxides. The composition of these mixed oxide inclusions will changeand their stability lowered during long-term use of the material, due tochanges in the surrounding matrix.

It has also been reported that an addition of a strongly oxide-formingelement in the form of titanium to a mechanically alloyed material thatcontains Y₂O₃ and 12% Cr can cause the separation of complex (Y+Ti)oxides, resulting in a material that has greater mechanical strengththan a material that contains no titanium. The strength at elevatedtemperatures can be further improved, by adding molybdenum.

Thus, a material that has good strength properties can be obtained bymeans of a mechanical alloying process.

Mechanical alloying, however, is encumbered with several drawbacks.Mechanical alloying is carried out batch-wise in high energy mills, inwhich the components are mixed to obtain an homogenous mixture. Thebatches are relatively limited in size, and the grinding processrequires a relatively long period of time to complete. The grindingprocess is also energy demanding. The decisive drawback with mechanicalalloying resides in the high product costs entailed.

A process in which an FeCrAl material alloyed with fine particles couldbe produced without needing to apply high energy grinding” would behighly beneficial from the aspect of cost.

It would be advantageous if the material could be produced by gasatomization, i.e., the production of a fine powder that is latercompressed. This process is less expensive than when the powder isproduced by grinding. Very small carbides and nitrides are precipitatedin conjunction with the rapid solidification process, such carbides andnitrides being desirable.

However, the titanium constitutes a serious problem when atomizing anFeCrAl material. The problem is that small particles of mainly TiN andTiC are formed in the smelt prior to atomization. These particles tendto fasten on the refractory material. Since the smelt passes through arelatively fine ceramic nozzle prior to atomization, these particleswill fasten to the nozzle and gradually accumulate. This causes cloggingof the nozzle, therewith making it necessary to disrupt the atomizationprocess. Such stoppages in production are expensive and troublesome.Consequently, FeCrAl materials that contain titanium are not produced byatomization in practice.

SUMMARY OF THE INVENTION

The present invention solves this problem and relates to a method inwhich an FeCrAl material can be produced by means of atomization.

The present invention thus relates to a method of producing an FeCrAlmaterial by gas atomization, wherein said material in addition to iron(Fe), chromium (Cr) and aluminum (Al) also contains minor fractions ofone or more of the materials molybdenum (Mo), hafnium (Hf), zirconium(Zr), yttrium (Y), nitrogen (N), carbon (C) and oxygen (O), and whereinthe method is characterized by causing the smelt to be atomized tocontain 0.05-0.50 percent by weight tantalum (Ta) and, at the same time,less than 0.10 percent by weight titanium (Ti).

The invention also relates to a high temperature material of a powdermetallurgical FeCrAl alloy produced by gas atomization. In addition tocontaining iron (Fe), chromium (Cr) and aluminum (Al), the material alsoincludes minor fractions of one or more of the materials molybdenum(Mo), hafnium (Hf), zirconium (Zr). yttrium (Y), nitrogen (N), carbon(C) and oxygen (O). The material also includes 0.05-0.50 percent byweight tantalum (Ta) and, at the same time, less than 0.10 percent byweight titanium (Ti).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method of producing an FeCrAlmaterial by gas atomization. In addition to iron (Fe), chromium (Cr) andaluminum (Al), the FeCrAl material also includes minor fractions of oneor more of the materials molybdenum (Mo), hafnium (Hf), zirconium (Zr),yttrium OO, nitrogen (N), carbon (C) and oxygen (O).

According to the present invention, the smelt to be atomized contains0.05-0.50 percent by weight tantalum (Ta) and also less than 0.10percent by weight titanium (Ti).

It has been found that tantalum imparts strength properties that arecomparable with those obtained when using titanium, and at the same timeTiC and TiN are not formed in quantities that cause clogging of thenozzle. This applies even when the smelt contains 0.10 percent by weighttitanium.

Thus, it is possible to produce the material in question by gasatomization, by using tantalum instead of at least a part of thetitanium quantity.

It is usual, and also possible, to use argon (Ar) as the atomizing gas.However, argon is adsorbed partly on accessible and available surfacesand partly in pores in the powder grains. In conjunction with subsequentheat consolidation and heat processing of the product, the argon willcollect under high pressure in microdefects. These defects swell to formpores in later use at low pressure and high temperature, therebyimpairing the strength of the product.

Powder that is atomized by means of nitrogen gas does not behave in thesame manner as argon, since nitrogen has greater solubility in the metalthan argon and since nitrogen is able to form nitrides. When gasatomizing with pure nitrogen gas, the aluminum will react with the gasand marked nitration of the surfaces of the powder grains can occur.This nitration makes it difficult to create bonds between the powdergrains in conjunction with hot isostatic pressing (HIP), causingdifficulties in the heat processing or the heat treatment of theresultant blank. In addition, individual powder grains may be sosignificantly nitrated as to cause the major part of the aluminum tobind as nitrides. Such particles are unable to form a protective oxide.Consequently, they can disturb the formation of oxide if they arepresent close to the surface of the end product.

It has been found that some oxidation of the powder surfaces is obtainedwhen a controlled amount of gaseous oxygen is supplied to the nitrogengas, while considerably reducing nitration at the same time. The risk ofoxide disturbances is also greatly reduced.

Consequently, in accordance with one highly preferred embodiment,nitrogen gas (N₂) is used as an atomizing gas to which a given quantityof oxygen gas (O₂) is added, said amount of oxygen gas being such as tocause the atomized powder to contain 0.02-0.10 percent by weight oxygen(O) at the same time as the nitrogen content of the powder is 0.01-0.06percent by weight.

According to one preferred embodiment, the smelt is caused to have acomposition in which the powder obtained has the following compositionin percent by weight, subsequent to atomization:

Fe balance Cr 5-25 percent by weight Al 3-7 Mo 0-5 Y 0.05-0.60 Zr0.01-0.30 Hf 0.05-0.50 Ta 0.05-0.50 Ti 0-0.10 C 0.01-0.05 N 0.01-0.06 O0.02-0.10 Si 0.10-0.70 Mn 0.05-0.50 P 0-0.08 S 0-0.005

According to one particularly preferred embodiment, the smelt is causedto have a composition such that subsequent to atomization the resultantpowder will have roughly the following composition in percent by weight:

Fe balance Cr 21 percent by weight Al 4.7 Mo 3 Y 0.2 Zr 0.1 Hf 0.2 Ta0.2 Ti <0.05 C 0.03 N 0.04 O 0.06 Si 0.4 Mn 0.15 P <0.02 S <0.001

Subsequent to heat treatment, the creep strength or creep resistance ofthe material is influenced to a great extent by the presence of oxidesof yttrium and tantalum and by carbides of hafnium and zirconium.

According to one preferred embodiment, the value of the formula((3×Y+Ta)×O)+((2×Zr+Hf)×(N+C)), where the identification of the elementsin the formula represents, the content in weight percent of therespective elements in the smelt, is greater than 0.04 but smaller than0.35.

Although the invention has been described above with reference to anumber of exemplifying embodiments, it will be understood that thecomposition of the material can be modified to some extent while stillobtaining a satisfactory, material.

The present invention is therefore not restricted to said embodiments,since variations can be made within the scope of the accompanyingclaims.

What is claimed is:
 1. A method of producing an FeCrAl material by gasatomization, said method comprising the steps of: adding to iron (Fe),chromium (Cr) and aluminum (Al) minor fractions of materials selectedfrom the group consisting of molybdenum (Mo), hafnium (Hf), zirconium(Zr), yttrium (Y), nitrogen (N), carbon (C) and oxygen (O), andcombinations and mixtures thereof, adding to a smelt to be atomized0.05-0.50 percent by weight tantalum (Ta) and less than 0.10 percent byweight titanium (Ti), and gas atomizing the smelt.
 2. A method accordingto claim 1, including the step of utilizing nitrogen gas (N₂) as anatomizing gas and adding a given amount of oxygen gas (O₂) to theatomizing gas, wherein said amount of oxygen gas is such that theatomized powder contains 0.02-0.10 percent by weight oxygen (O) and0.01-0.06 percent by weight nitrogen (N).
 3. A method according to claim1, wherein the powder obtained after atomization has the followingcomposition in percent by weight: F balance Cr 15-25 Al 3-7 Mo 0-5 Y0.05-0.60 Zr 0.01-0.30 Hf 0.05-0.50 Ta 0.05-0.50 Ti 0-0.10 C 0.01-0.05 N0.01-0.06 O 0.02-0.10 Si 0.10-0.70 Mn 0.05-0.50 P 0-0.08 S 0-0.005.


4. A method according to claim 3, wherein the smelt has a compositionsuch that the powder obtained after atomization has substantially thefollowing composition in percent by weight: Fe balance Cr 21 Al 4.7 Mo 3Y 0.2 Zr 0.1 Hf 0.2 Ta 0.2 Ti <0.05 C 0.03 N 0.04 O 0.06 Si 0.4 Mn 0.15P <0.02 S <0.001.


5. A method according to claim 1, wherein the value of the formula((3×Y+Ta)×O)+((2×Zr+Hf)×(N+C)), in which the elements are given inpercent by weight in the smelt, is greater than 0.04 and less than 0.35.6. High temperature material of a powder metallurgical FeCrAl alloyproduced by gas atomization, said material comprising: iron (Fe),chromium (Cr) and aluminum (Al) and minor fractions of materialsselected from the group consisting of molybdenum (Mo), hafnium (Hf),zirconium (Zr), yttrium (Y), nitrogen (N), carbon (C) and oxygen (O),and combinations and mixtures thereof, and wherein the material includes0.05-0.50 percent by weight tantalum (Ta) and less than 0.10 percent byweight titanium (Ti), said tantalum and titanium present in amounts suchthat titanium compounds, are not formed in quantities that causeclogging of a gas atomization nozzle.
 7. High temperature materialaccording to claim 6, wherein the powder obtained by gas atomization hasthe following composition in percent by weight: Fe balance Cr 15-25 Al3-7 Mo 0-5 Y 0.05-0.60 Zr 0.01-0.30 Hf 0.05-0.50 Ta 0.05-0.50 Ti 0-0.10C 0.01-0.05 N 0.01-0.06 O 0.02-0.10 Si 0.10-0.70 Mn 0.05-0.50 P 0-0.08 S<0.005.


8. High temperature material according to claim 7, wherein the powderobtained has substantially the following composition in percent byweight: Fe balance Cr 21 Al 4.7 Mo 3 Y 0.2 Zr 0.1 Hf 0.2 Ta 0.2 Ti <0.05C 0.03 N 0.04 O 0.06 Si 0.4 Mn 0.15 P <0.02 S <0.001.


9. High temperature material according to claim 6, wherein the value ofthe formula ((3×Y+Ta)×O)+((2×Zr+Hf)×(N+C)), in which the elements aregiven in percent by weight of a smelt, exceeds 0.04 but is less than0.35.